CHLOROFLUOROCARBON UPTAKE IN A WORLD OCEAN MODEL .2. SENSITIVITY TO SURFACE THERMOHALINE FORCING AND SUBSURFACE MIXING PARAMETERIZATIONS

Part 1 of this study [England et al., 1994] examined the sensitivity o f simulated oceanic chlorofluorocarbon (CFC) to changes in the way the air-sea gas exchange rate is parameterized in a World Ocean model. In part 2 we consider more closely the role of surface thermohaline forc ing and subsurface mixing parameterizations in redistributing CFC-11 a nd CFC-12 in the ocean. In particular, a series of five different mode l ocean experiments are forced with the same air-sea CFC flux paramete rization. The five cases include (1) a control run with a standard sea sonal cycle in surface forcing and traditional Cartesian mixing, (2) a run in which the production rate and salinity of Antarctic Bottom Wat er (AABW) is increased, (3) a run in which the production, outflow rat es, and density of North Atlantic Deep Water (NADW) is increased, (4) a run with enhanced isopycnal mixing of passive tracers, and finally ( 5) a run in which the effects of eddies on the mean ocean flow are par ameterized following Gent and McWilliams [1990]. The simulated CFC upt ake in the Southern Ocean far exceeds observations in the first four e xperiments. The excessive uptake is linked to the poor model simulatio ns of Southern Ocean deep water masses, where, for example, model Circ umpolar Deep Water is typically 0.2 to 0.4 kg m(-3) too buoyant. The i nsufficient density of the deep water allows for extensive penetration of convective adjustment to great depth during winter, in contrast to observations, and this results in excessive downward mixing of the CF C-enriched surface waters. Compared with the control experiment, the S outhern Ocean CFC uptake is reduced in the cases with increased AABW s alinity and NADW density, as a result of slightly higher deep water de nsity and reduced wintertime convection in those experiments. Neverthe less, CFC uptake in the Southern Ocean still substantially exceeds obs erved ocean CFC content in the adjusted surface forcing cases. The mos t extreme uptake occurs in case 4, where, in addition to deep convecti ve mixing of CFC, there is also mixing into the ocean interior along i sopycnal surfaces having an unrealistic orientation. The Southern Ocea n CFC uptake in case 5, using the mixing scheme of Gent and McWilliams [1990], is dramatically reduced over that in the other runs. Only in this run do deep densities approach the observed values, and wintertim e convection is largely suppressed south of the Antarctic Circumpolar Current. Deep penetration of CFC-rich water occurs only in the western Weddell and Ross Seas. This run yields CFC sections in the Southern O cean which compare most favourably with observations, although substan tial differences still exist between observed and simulated CFC. The s imulation of NADW production is problematic in all runs, with the CFC signature indicating primary source regions in the Labrador Sea and im mediately to the southeast of Greenland, while the Norwegian-Greenland Sea overflow water (which is dominant in reality) plays only a minor role. Lower NADW is insufficiently dense in all runs. Only in the run with surface forcing designed to enhance NADW production does the CFC signal penetrate down the western Atlantic boundary in a realistic man ner. However, this case exhibits an unrealistic net ocean surface heat loss adjacent to Greenland and so cannot be advocated as a technique to improve model NADW production. Conventional depth sections and volu metric maps of CFC concentration indicate that on the decadal timescal es resolved by CFC uptake the dominant determining factor in overall m odel ventilation is the choice of subsurface mixing scheme. The surfac e thermohaline forcing only determines more subtle aspects of the subs urface CFC content. This means that the choice of subgrid-scale mixing scheme plays a key role in determining ocean model ventilation over d ecadal to centennial timescales. This has important implications for c limate model studies.